Bendable stent

ABSTRACT

A stent includes a matrix of struts. The matrix of struts may be disposed parallel to one another in a non-expanded disposition. A plurality of loops along the longitudinal axis may be formed by the matrix of struts. In one aspect, a plurality of cusps in each loop of the plurality of loops connects adjacent struts, the plurality of cusps including tied cusps and free cusps. The tied cusps of one loop can be connected to the tied cusps of an adjacent loop by a bridge extending therebetween. In one aspect, the adjacent struts connected by a tied cusp have different lengths. In one aspect, at least one free cusp in each loop joins two struts of approximately equal length.

PRIORITY

This application is a continuation of U.S. patent application Ser. No.12/594,531, filed Dec. 7, 2009, now U.S. Pat. No. 8,518,101, which is aU.S. national stage application under 35 USC §371 of InternationalApplication No. PCT/EP2008/054007, filed Apr. 3, 2008, claiming priorityto United Kingdom Patent Application No. 0706499.1, filed Apr. 3, 2007,each of which is incorporated by reference in its entirety into thisapplication.

FIELD OF THE INVENTION

This invention relates to radially expansible stents for transluminaldelivery to a stenting site within the body of a patient, the stenthaving an enhanced capacity for bending, after deployment in the body.

There are some stenting sites within the body in which there issubstantial deformation of the lumen that is stented. Consider, forexample, a peripheral vascular stent at a site near the knee. When anexpanded stent suffers severe bending, there can be buckling on theinside of the bend. Even before there is any catastrophic buckling, thelikelihood exists that portions of the stent matrix, spaced apart alongthe axis of the stent lumen, will approach each other and impact, on theinside of any temporary tight bend, to the detriment not only of thetissue caught between the impacting portions of the stent, but also thestent matrix itself. It is an object to the present invention toameliorate this problem.

BACKGROUND OF THE INVENTION

The present applicant is a specialist in the manufacture of stents ofnickel-titanium shape memory alloy, manufactured from a raw materialthat is a tubular workpiece of that alloy. To make the stent matrix, thealloy tube workpiece is subjected to a laser-cutting step in which thelaser cuts a multiplicity of slits in the tubular workpiece. Each slitextends through the entire wall thickness of the tube, and for the mostpart, the slits all have the same length and are all parallel to thelongitudinal axis of the tubular workpiece. When one advances around thecircumference of the tubular workpiece, crossing transversely over amultiplicity of the slits, one by one, alternate slits that one crossesare staggered, in the axial direction of the tube, by a distance that isaround half the length of each slit. When such a slitted tube is slippedover a mandrel, and expanded radially, each slit opens out into adiamond-shaped aperture in the wall thickness of the tube. Looked at inanother way, the creation of the slits at the same time creates strutsof material that lie between adjacent slits, and the struts in theradially expanded tube emerge as zig-zag stenting rings with acharacteristic strut length within any one zig-zag ring that is more orless half the length of each of the slits cut by the laser.

Where two struts, next adjacent within the circumference of a zig-zagring, come together, we can call this a “cusp”. The cusps of eachzig-zag ring are contiguous with cusps of the next adjacent stentingring.

For enhanced flexibility of the zig-zag stent matrix, many of the“connector portions” between facing cusps of adjacent zig-zag stentingrings can be parted, to leave only a few (typically four or less)connector bridges between any two axially adjacent zig-zag stentingrings. See our WO 94/17754. The surviving connector bridges have alength direction parallel to the longitudinal axis of the stent matrix.

However where these connector bridges have been removed, there are stillcusps of adjacent zig-zag stenting rings that are effectively “head tohead” across the narrow gap with a cusp belonging to the adjacentzig-zag ring. When such a narrow gap is on the inside of the bend, uponbending the expanded stent (by movement of the body after the stent hasbeen placed in the body), there is the likelihood of the two cusps headto head impacting on each other. It is common to call this “peak topeak”.

In this discussion, it is important to distinguish between the radiallycompact trans-luminal delivery disposition of the stent matrix (not verydifferent from the as-cut disposition of the stent matrix, beforeexpansion on the mandrel to diamond-shaped apertures) and the radiallyexpanded and deployed configuration of the stent, where the struts formzig-zag rings. A head to head facing configuration of parted connectorportion cusps is tolerable for the delivery procedure but to be avoided,if that is feasible, after stent deployment and radial expansion.

The present applicant has been interested in this objective for someyears. For a previous proposal for improvements see its WO 01/76508/published Oct. 18, 2001. The present invention represents a freshapproach to the problem and, it is thought, a more elegant solution.

Other makers of stents have concerned themselves with the sameobjective. See for example US 2004/0073290 A1 where {paragraph 0002) itis explained that “if adjacent rings are spaced too close together” then“interference can occur between adjacent rings on the inside of a bend”.Clearly, the idea of spacing the axially adjacent rings further aparthas limited appeal/because it leaves the space between the ringsunstented.

Self-expanding stents of nickel-titanium shape memory alloy are notparticularly radiopaque and so are often equipped with radiopaquemarkers, of which one favoured material is tantalum because it is closeto the nickel-titanium alloy in electrochemical potential/therebyminimising galvanic corrosion in the electrolyte of a bodily fluid.

Self-expanding stents are usually deployed by proximal withdrawal of asheath of a catheter delivery system. To prevent the stent movingproximally with the withdrawing sheath it is conventional to use apushing annulus, that abuts the proximal end zone of the stent andresists any proximal movement of the stent relative to the stentdelivery catheter as such. As stent performance and length go up so doesthe compressive stress imposed on the end zone of the stent by thepushing annulus during withdrawal. It is important to avoid imposing onany part of the end zone a magnitude of stress higher than that of thedesign performance limits for that stent. The present inventor knowsthat one way to manage that peak stress is to build the stent so thatthe end zone has all its cusps touching a notional circle transverse tothe longitudinal axis of the stent, so that the stress from the pushingannulus is shared equally amongst all those cusps. For an example of astent with such an end zone, see WO 2006/047977.

EP-A-1767240 offers ways to increase the flexibility of a stent in itsradially compact delivery disposition. It suggests resorting to portionsnot parallel to the stent length, such as struts that are curved, orbridges that are skewed to the long axis of the stent.

SUMMARY OF THE INVENTION

The present invention is defined in the claims below in which differentaspects are presented in respective independent claims, and dependentclaims are directed to optional or preferred features. In oneembodiment, the invention takes the form of a laser-cut stent in whichthe slits cut by the laser in the tubular workpiece that is theprecursor of the stent are staggered with respect to each other, in thelength direction of the stent cylinder such that the slits cut by thelaser are, in general, all the same length but the struts created bycutting the laser slits are not all the same length because the axialstagger between circumferentially adjacent slits is arranged to besomething other than half the common slit length. However, as theaccompanying drawings will reveal, even when many of the slits can bemade free of axial displacement (staggering) relative to thecircumferentially next adjacent slits, the effect of eliminating head tohead cusps on adjacent stenting rings can still be accomplished. As longas some of the adjacent slits are staggered, by an amount other than aone half slit length, the necessary circumferential displacement offacing cusps away from each other can still be achieved, as the slittedtube undergoes radial expansion.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a view from above, of a slitted tube opened out flat

FIG. 2 shows a portion of the matrix of FIG. 1, radially expanded (butalso opened out flat)

FIG. 3 shows another embodiment of slitted tube opened out flat and

FIG. 4 shows the FIG. 3 strut matrix, opened out flat, and expanded, and

FIG. 5 is a perspective view of the FIG. 4 strut matrix, not opened outflat

FIG. 6 is a view of another slitted tube opened out flat and

FIG. 7 is a view of the slitted tube of FIG. 6, radially expanded andopened out flat and

FIG. 8 is a perspective view of the radially expanded tube of FIGS. 6and 7; and

FIG. 9 is a view from above, like that of FIGS. 1, 3 and 6 but of yetanother embodiment of a slitted tube opened out flat.

DETAILED DESCRIPTION

Referring to FIG. 1, we see a slitted tube 10, opened out flat byparting the slitted tube at interface portions 12, 14 and 16 to display,opened out flat, a succession of stenting rings I, II, III, IV arrangednext to each other along the length of the slitted tube parallel to itslong axis direction X. Each of the four stenting rings exhibits a serialprogression of n_(t) struts, here 24 struts, (20) separated from eachother by the slits through the wall thickness of the tubular workpiece,the succeeding struts of each stenting ring being joined by successivecusps 24. In the unexpanded slitted configuration of FIG. 1, each cuspis in “head-to-head” relationship, along the axis direction X of theslitted tube, with a cusp of the adjacent stenting ring. As can be seen,each stenting ring is connected to the next adjacent stenting ring byfour bridges 26 distributed at regular intervals (90°) around thecircumference of the slitted tube. The number of bridges per ring isn_(s) and the number of struts between successive bridges is n_(s) so:n_(t)=_(s)•n_(b).

In stent technology, particularly stents made of shape memory alloy(NITINOL), a strut matrix made by slitting a precursor tube isconventional.

Turning to FIG. 2, we see a portion of the FIG. 1 slitted tube radiallyexpanded so that the struts of each stenting ring are inclined to theaxial direction X and present themselves as a zig-zag sequence of strutsaround the circumference of the stent. It will be noted that the cusps24 of adjacent stenting rings are still in head-to-head disposition.Skilled readers will appreciate that any gross bending of a deployedstent is liable to bring opposing cusps on the inside of the bend intophysical contact with each other.

Turning to FIG. 3, we can recognise the same pattern of 24 struts 20making up 4 adjacent stenting rings I, II, III, IV, recognisablyequivalent to what is shown in FIG. 1. Further, just as in FIG. 1, eachcusp 24 is in head-to-head relationship with a cusp of the next adjacentstenting ring. Just as in FIG. 1, each stenting ring is connected to thenext adjacent stenting ring by four bridges 26.

However, the slits 22 in the tube 10, that have created the strutmatrix, are axially staggered relative to each other, in a way which isnot present in drawing FIG. 1. In consequence of this axial staggering,there is also axial staggering of the gaps 30 between each pair offacing cusps 24. In FIG. 3, there is shown a greater axial separationbetween facing cusps 24 than is apparent from FIG. 1, but this is notthe decisive difference between the FIG. 1 concept and that of FIG. 3.

Reverting to FIG. 1, and concentrating on a pair of struts definingbetween them an individual gap 22, one can see that the axial length ofthe two struts, one each side of the slit 22 1 is the same. However,when we look at FIG. 3, and a particular slit 22, we notice that thelength of the strut that extends down each side of the slit 22, from thecommon cusp 24 at one end of the slit, are different. This hasrepercussions for the way the struts deform when the slitted tube ofFIG. 3 is radially expanded, to the zig-zag pattern shown in FIG. 4.

Comparing FIG. 4 with FIG. 2, it is immediately evident that there areno longer pairs of cusps 24 facing each other, head to head. Instead,each cusp points towards a gap between two adjacent cusps of theadjacent zig-zag stenting ring. The skilled reader will appreciate thatwhen the stent of FIG. 4 is bent (into a banana shape) each cusp is freeto advance axially into the gap between two adjacent cusps of theadjacent stenting ring, rather than striking, head on, the facing cuspof the adjacent stenting ring, as in FIG. 2.

FIG. 5 is a perspective view but shows the same phenomenon as is drawnin drawing FIG. 4 I with the same strut matrix.

The skilled reader will grasp that the number of struts in each stentingring need not be 24, and the number of bridges between adjacent stentingrings need not be four. Another arrangement that shows promise is one inwhich each stenting ring has 42 struts and adjacent stenting rings areconnected by three bridges distributed at 120° intervals. Such anarrangement is shown in FIG. 9 1 and is described below.

FIGS. 6, 7 and 8 show another attractive design, namely, a slitted tubewith 40 struts per ring and four bridges. Since other aspects of thedesign are described above with reference to FIGS. 3 and 4, the samereference numbers are used to identify corresponding features. of thedesign. Again, it can be seen that when the FIG. 6 slitted tube isopened out radially, the cusps 24 automatically move to positions wherethey are no longer facing head to head any cusp of the adjacent zig-zagstenting ring, with consequential advantages of avoiding cusp to cuspcontact when the deployed stent is subjected to bending deformation.

In FIG. 6, in loop III, three successive bridges are labeled B1, B2, B3.Bridges Bland B3 connect loop III to loop IV. Bridge B2 is one of thefour bridges that connect loop III. Between bridges B1 and B2, andbetween bridge B2 and B3, is a sequence of five struts. Three of thesestruts S1, S2, S3, have the same length. Each extends between two freecusps. The other two struts, S4 and S5, have lengths different from eachother. This length difference is what takes the free cusps of adjacentloops out of a head-to-head facing relationship in the expandedconfiguration of the stent, as can be understood from FIGS. 7 and 8,which also reveal that the bridges are correspondingly skewed, relativeto the long axis of the stent, in the expanded disposition of the stent.

The lengthwise staggering of cusps that characterises the presentinvention can deliver useful technical effects that include thefollowing.

When a self-expanding strut is to be released from its catheter deliverysystem, the usual way is to withdraw proximally, relative to the stent,a restraining sheath that surrounds its abluminal surface. When allcusps in a loop are at the same point along the axis of the stent, allcan spring radially outwardly from the sheath simultaneously. Thisimpulsive release is not ideal for controlled release. Axial staggeringof cusps can assist in releasing the stent more progressively andsteadily, cusps escaping one by one from the inward radial confinementof the proximally retreating sheath.

For some stents, the design features non-identical proximal and distalends, so that it is critically important to load the stent in thedelivery system with its distal end nearer the distal end of thedelivery system. An advantage of the present invention is that itpermits the building of stents with identical distal and proximal ends,that are indifferent to the choice of stent end to lie closer to thedistal end of the delivery system.

The axial staggering opens up possibilities for “recesses” such asrecesses 40 in FIG. 3, where radiopaque marker elements 50 can belocated. These elements thus lie snug between circumferentially spacedapart cusps 42, 44 and axially adjacent to intervening cusps 46, towhich it will be convenient to attach the marker. Any axial pushing onthe stent, while the confining sleeve is withdrawn is customarilyapplied to the end surface of the stent. By locating markers in the endrecesses and arranging for the end elevation of the stent to compriseboth cusps and markers, the stresses on the end elevation aredistributed around the circumference as evenly as possible, and over themaximum area of surface of the implant, which is good for fatigueperformance, quality control, and efficiency of stent release. Finally,with markers recessed into the end zone of a stent, the markers whenimaged give a true impression of where the stent matrix is, and where itis not. A short look at US 2006/0025847 serves to reveal the advantagesof the present proposal over another recent proposal to deal withpushing forces.

Not to be underestimated is the advantage yielded by this invention,that a “peak-to-valley” distribution of cusps in the expanded deployeddisposition is automatic, regardless how short are the bridges betweenadjacent stenting loops. Short, strong, robust bridges that connectaxially adjacent stenting loops are greatly to be welcomed, for manyreasons. In particular/they are less vulnerable to inadvertent straining(bad for fatigue performance if nothing else) when stent matrices arebeing installed in a catheter delivery system, or when being deployedout of one. Put another way, the stent with short stubby bridges can berated for greater loads imposed on it during loading or deployment.Since the radial force that a stent can exert on surrounding bodilytissue increases with the number of stenting loops per unit (axial)length of the stent, a reduction in the length of the bridges connectingaxially adjacent stenting loops will give rise to an increased stentingforce.

However, short stubby bridges are disadvantageous, to the extent thatthey prejudice stent flexibility. The more flexible a stent is, thebetter its resistance to fatigue failure (other things being equal). Oneway to deliver more flexibility, despite an absence of much flexibilityin the bridges, is to increase the number of struts in the sequence ofstruts between each bridge and the next bridge. On that basis, thearrangement of FIG. 9, with 7 struts between any two bridges B1, B2 orB2, B3, is superior to the FIG. 6 design with 5 struts, itself superiorto that of FIG. 3, with 3 struts.

When it comes to radiopaque markers, it is important to arrange themarkers so that they are distributed around the circumference of thestent, in the radially compact delivery disposition of the stent, asevenly as is practicable. In FIG. 3, the arrangement is even. FIG. 6shows one possible arrangement of tantalum markers 60, 62, 64, 66 whichis not far from an even distribution in the compact form of the stent(although further from evenly distributed when the stent is expanded).In the FIG. 9 design it is clear that each end of the stent offers onlythree recesses for installation of a set of three markers evenlydistributed around the circumference of the stent.

The markers can be of different shapes, in order to meet these designobjectives, as is illustrated in FIG. 6, as one example.

One thing that is striking about the present invention is how itdelivers a simple pattern of linear slits in the compact configurationthat exhibits in each stenting loop a sequence of stepwisedisplacements, up and down the axis of the stent, in the positions ofthe free cusps, yet, in the expanded disposition of the stent, the axialsteps are gone. Instead, the bridges are skewed, and the free cusps arecircumferentially displaced, relative to the free cusps of the adjacentstenting loop that were facing them, head-to-head, in the compactdisposition. Of significance is that, in the expanded disposition, whenthe stent must exert radially outward stenting force on the bodilytissue that forms the wall of the stented bodily lumen, the zig-zagstruts of each stenting rings march around the circumference of thelumen in a progression in which axial displacement of free cusps,relative to each other, is difficult to discern. Instead, the stentingloops deploys in a way that is close to an optimal planar hoop,transverse t the axis, for generating a large mechanical radiallyoutward stenting force.

Applicant's WO 2007/135090 discloses a stent that is “bend-capable” inthat cusps move out of a “head-to-head” facing relationship in theexpanded deployed stent, when the stent tube is bent out of a straightconfiguration. It will be apparent to the skilled reader that thepresent invention (lengthwise staggering of cusps) can be combined withthe invention of WO 2007/135090 (skewed unit cell) to deliver a stentmatrix that avoids a head to head facing relationship of cusps,regardless of the extent to which the stent is bent out of a straightline after deployment. One way to accomplish the result explained in WO2007/135090 is to arrange the strut matrix such that n 8/2 is an evennumber.

It hardly needs to be added, that the stents taught in this disclosurecan be used in the same way as prior art stents are used. They can carrygraft material, or drugs, for example. They can be deliveredtransluminally, by a suitable catheter delivery system. They can carryradiopaque markers, as is taught in the state of the art. They will findparticular application in situations where the stent, after deployment,is subject to a high degree of bending.

The present drawings show specific embodiments which are to be assessedas exemplary, not limiting. The stent need not be made from shape memorymetal and need not be laser cut. The inventive concept disclosed hereinis applicable to a wide range of known stent technologies.

What is claimed is:
 1. A stent comprising a matrix of struts disposedparallel to one another comprising: a plurality of loops disposed alonga longitudinal axis of the stent comprising: end-zone loops composed oftied cusps, free cusps and radiopaque markers; and intermediate-zoneloops composed of tied cusps and free cusps, wherein tied cusps and freecusps of end-zone loops and intermediate-zone loops connect adjacentstruts; and a bridge connecting a tied cusp of one loop head-to-headwith a tied cusp of an adjacent loop wherein all of the free cusps onone end of the end-zone loop and all of the radiopaque markers of theend-zone loop are disposed to touch a notional circle transverse to thelongitudinal axis of the stent such that the stress imposed on the stentby a pushing annulus is shared among all of the free cusps and all ofthe radiopaque markers on the one end of the end-zone loops; wherein theradiopaque markers are distributed around the circumference of theend-zone loops, each of the markers positioned in a gap defined betweenouter surfaces of adjacent end cusps; and wherein the adjacent strutsconnected by a tied cusp have different lengths, and at least one freecusp in each loop joins two struts of approximately equal length.
 2. Thestent according to claim 1 wherein free cusps of adjacent loops arecircumferentially displaced from one another.
 3. The stent according toclaim 2, wherein each intermediate loop includes either 24, 40, or 42struts.
 4. The stent according to claim 3 wherein the stent includes 40struts in each loop and 4 bridges connecting each pair of adjacentloops, and at least three consecutive struts having approximately equallengths located between adjacent bridges.
 5. The stent according toclaim 3 wherein the stent includes 42 struts in each loop and 3 bridgesconnecting any two loops, and at least five consecutive struts havingapproximately equal lengths located between adjacent bridges.
 6. Thestent according to claim 2 wherein the bridges between any two adjacentintermediate-zone loops are evenly distributed around the circumference.7. The stent according to claim 6 wherein radiopaque markers in an endzone of the stent are evenly spaced around the circumference.
 8. Thestent according to claim 7 wherein at least two radiopaque markers haveshapes different from each other.
 9. The stent according to claim 8wherein at least three radiopaque markers each have a different shapefrom one another.
 10. The stent according to claim 1 wherein there are Nstruts between two successive bridges connecting two adjacent loops Aand B, N being an integral even number, and N/2 being an integral oddnumber.
 11. The stent according to claim 1 wherein there are N strutsbetween successive bridges connecting two adjacent loops A and B, Nbeing an integral even number, and N/2 being also an integral evennumber.
 12. The stent according to claim 1 wherein at least threeradiopaque markers have a different shape from one another.